The long-term goal of the project is to substantially improve our understanding of the interaction between bone and muscle. The ?mechanostat theory? proposes that bone adapts its morphology and strength to long-term loads exerted by muscle contraction (1). However, the bidirectional signaling between muscle and bone that has emerged broadens the relationship beyond that of a purely mechanical perspective (2). We propose to examine the association between maximal capacity of muscle to generate ATP (ATP-max) by 31P-Magnetic Resonance Spectroscopy (31P ?MRS), quadriceps contractile volume by MRI, total muscle mass (TMM) by D3 creatine dilution (D3Cr), and functional muscle power and bone microarchitecture and strength. The underlying scientific premise is that age related declines in the capacity to generate ATP in muscle, muscle volume, TMM and functional power lead to decreases in bone strength and poor bone microarchitecture. This proposal builds on the Study of Muscle, Mobility and Aging (SOMMA), the first prospective study of muscle aging aimed to study the contributions of skeletal muscle biology and function to major mobility disability. We propose to extend SOMMA to include important skeletal measures of bone strength using high-resolution peripheral quantitative computed tomography (HRpQCT). To our knowledge, no studies have linked ATP-max and quadriceps contractile volume to volumetric BMD, microarchitecture and strength (failure load as assessed by finite element analysis). We will examine these characteristics globally and separately in the trabecular and cortical compartments and in a weight bearing (tibia) and non-weight bearing (radius) bone site. The D3Cr method of measuring TMM is limited to men enrolled in the Osteoporotic Fractures in Men study (MrOS; mean age 84 yrs). In SOMMA we will be able to study the D3Cr TMM associations with the HRpQCT parameters on a wider age group of both men and women. Establishing HRpQCT relationships to TMM and functional power from the stair climb may identify target outcomes for interventions that can improve both tissues (2). We will recruit 400 of the 438 SOMMA subjects in Pittsburgh. Our proposal offers a cost-efficient opportunity to investigate the bone-muscle interaction in older adults using state-of-the-art measurements. We propose the following specific aims: Aim 1: To determine the association of bone microarchitecture, volumetric BMD, and bone strength (failure load) globally and in the trabecular and cortical compartments separately to in-vivo ATP- max, contractile muscle volume and total skeletal muscle mass. Aim 2: To determine the association between functional muscle power as measured by the stair climb to HRpQCT parameters of volumetric density, microarchitecture and strength. Aim 3, Exploratory: To explore sex differences in the associations studied in Aims 1 and 2. OVERALL IMPACT: This SOMMA ancillary study will be the first to study novel properties of muscle biology, volume, and functional power and their relationships to bone strength in a well- characterized population of older men and women.